Translation ‘this field is required’ missing

Hi,
Something very strange to me happens with ‘this field is required’ text and similar alert on Contactform7 fields.
My site is in Italian, Contactform7 shows italian backend language, Conditional Field shows ‘errors’ in italian, but Contactform itself shows ‘english text’ error alerts when submit form.
So for any ‘conditional gruop’ part of the form I can read ‘campo richiesto’ (italian), but things like Checkbox is still ‘This field is required’ (english).
So I have a double language form.

I tried to look at plugin language file and it’s ok. In languages/plugi/contact-form-7-it_IT.po and mo Italian translation is ok.
I also download latest italian translation. But nothing to do.
here the screen shot: https://www.supergap.it/screenshot-contactform.png

Could you please help me with this issue.
Thanks a lot!